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Allelopathic effects of Ambrosia artemisiifolia L. in the invasive process
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DOI:10.1016/j.cropro.2013.08.009 Terms of use:
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1
Short title: Allelopathy of Ambrosia artemisiifolia
1Potential allelopathic effects of Common ragweed (Ambrosia
2
artemisiifolia L.)
3
Francesco Vidotto, Franco Tesio and Aldo Ferrero
45
Abstract 6
Ambrosia artemisiifolia (common ragweed), an annual native to North America, is now
7
present in many European countries where it causes summer hayfever and disturbs 8
several important crops. We investigated if common ragweed invasiveness could be 9
explained by its leaf tissue and root exudate alleopathic potential on indicator crops 10
(alfalfa, barley, corn, lettuce, tomato, and wheat), weeds (barnyardgrass, black 11
nightshade, common purslane, large crabgrass), and common ragweed itself. Different 12
residue substrates were prepared for soil incorpration and trials were conducted under 13
both laboratory (1, 2, and 3 g residues /Parker dish) and greenhouse conditions (1.28 g 14
residues / pot). The effect of the preparations on the germination and growth of the 15
indicator crops and weeds were evaluated relative to soil previously used to cultivate 16
common ragweed. 17
Results showed tomato was the most sensitive indicator crop species as growth was 18
reduced by more than 50% in both laboratory and greenhouse experiments. Lettuce crop 19
root and shoot growth were also inhibited, but only when common ragweed residues, and 20
not root exudates, were added to the substrate. Among the weeds, E. crus-galli was not 21
affected by common ragweed while D. sanguinalis suffered a large germination reduction 22
(90%) after incorporation of 3 g of residues. 23
2
Nomenclature: alfalfa, Medicago sativa L.; barley, Hordeum vulgare L.; barnyardgrass, 25
Echinochloa crus-galli (L.) Beauv. ECHCG; black nightshade, Solanum nigrum L. SOLNI;
26
common purslane, Portulaca oleracea POROL; corn, Zea mays L.; Common ragweed, 27
Ambrosia artemisiifolia L.; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; lettuce,
28
Lactuca sativa L.; pea, Pisum sativum L.; redroot pigweed,; tomato, Lycopersicon
29
esculentum Mill.; wheat, Triticum aestivum L.;
30 31
Key words: Phitotoxicity; residue degradation; crop rotation, plant invasion, root exudates. 32
33
Introduction 34
Common ragweed (Ambrosia artemisiifolia L.) is a herbaceous, annual plant that is native 35
to North America and a pioneer dominant in abandoned croplands in several areas of the 36
United States. This species is often present as a weed in field crops such as maize, 37
soybean, and wheat (Bassett and Crompton 1979; Fumanal et al. 2008) and may grow 38
exceptionally dense in fields plowed during the spring and subsequently abandoned 39
(Raynal and Bazzaz 1975). It is also a common weed found along roadsides, in urban 40
empty lots, and in other disturbed habitats (Lavoie et al. 2007). First reported in botantical 41
gardens in Europe during the latter half of the 1800s, common ragweed spead to several 42
European countries by the beginning of the 1900s (Chauvel et al. 2006; Vogl et al. 2008). 43
Many current objections to A. artemisiifolia center on its powerful provocation of pollen 44
allergies (Wayne et al. 2002). The plant flowers in the northern hemisphere from mid-45
August until cooler weather arrives during with each plant can produce a great number of 46
pollen grains (Rogers et al. 2006). The high allergenic potential coupled with its broad 47
spread has caused health organizations to name common ragweed one of the most 48
problematic invasive plants (Wayne et al. 2002). 49
3
Even though Ambrosia artemisiifolia is non-indigenous to several European plant 50
communities, little is known about the factors that characterize its invasive processes: 51
(Rabitsch and Essl 2006) its abiotic-limiting factors, the potential activity of its competitors, 52
and its natural enemies. One such theory, the “enemy release hypothesis” (Keane and 53
Crawley 2002), postulates that invasiveness can result from a loss of natural enemies 54
during the invasion process while another, known as the “novel weapon allelopathy” theory 55
(Callaway and Ashehoug 2000), suggests that plant community evolution is speeded when 56
subjected to allelo-chemicals produced by species that have not co-evolved (Warwick 57
1991). Fortunately, the allelopathic potential, defined as the suppressive activity of one 58
plant species on its neighbor plant(s) by toxic compound release, of the Asteraceae family 59
has been widely studied. In particular, the sensitivity of cereals seeded after crop species 60
belonging to this family (common sunflower) has largely been documented (Leather 1983). 61
Agricultural management programs that incorporate allelopathic plant residues through 62
rotational or cover crops is reported to have a positive environmental effect and is often 63
practiced by producers interested in reducing chemical usage (Tesio and Ferrero 2010). 64
A potential negative consequence of allelopathy is the production of toxic compounds by 65
non-native invasive species that unfavorably affect native communities (Vivanco et al. 66
2004). Noxious weeds may have an effect on other species through competition or the 67
release of growth inhibitors. In this situation, native plants are unable to tolerate 68
compounds released by a non-native plant that has not co-evolved in the same 69
environment (Hua et al. 2005). This process of environmental modification causes 70
continual change to the relationship and composition among the species in an area until a 71
new equilibrium and a stable community is established. 72
Common ragweed is present throughout the northern part of Italy, where it is considered 73
an annual summer crop weed after existing for decades in urban or disturbed areas and at 74
field edges (Patracchini et al. 2011). Recently, this species has been reported to occur 75
4
after winter cereal field harvest, even if common ragweed seedlings have already emerged 76
into the cereal crop. After crop harvest, when light conditions and resource availability are 77
favorable for growth, plants are able to effect large amount of canopy, flower, and produce 78
seeds (Patracchini et al. 2011). 79
This study was designed to evaluate the potential phytotoxic effects of common ragweed 80
upon succeeding crops which are common in Italian field rotations. Experiments were 81
performed in both laboratory and greenhouse studies conducted in Italy. Using controlled 82
laboratory and greenhouse conditions, we assessed the impact of A. artemisiifolia upon 83
the germination and growth behavior of several crop and weed species after incorporating 84
common ragweed dried residues into the soil. We also attempted to simulate common field 85
conditions by performing greenhouse studies in a controlled environment to elucidate the 86
effects of root exudates upon plant growth. 87
88
Materials and Methods 89
Plant material 90
The study was conducted during 2007 – 2008 in the laboratory and greenhouse of the 91
Dipartimento di Agronomia Selvicoltura e Gestione del Territorio at Grugliasco (Turin, Italy 92
– 45°03’53’’N 7°35’38’’E). Ambrosia artemisiifolia shoots and seeds were harvested during 93
2007 from a field heavily infested by the weed within the experimental site. Aboveground 94
tissues were collected in June 2007 from healthy individuals by cutting plants 10 cm above 95
the soil surface. The leaves were immediately separated from the stalks and dried in open 96
trays in the laboratory oven at 60°C. The resulting dried material was stored in tightly 97
closed plastic bags until needed in the experiment. Seeds were harvested by hand the 98
during the subsequent September and dried in open trays in the laboratory at room 99
temperature (22-25°C). Dried seeds were stored until needed in the refrigerator at +4°C. 100
5
The indicator crops included in the experiment were alfalfa (Medicago sativa L.), barley 101
(Ordeum vulgare L.), lettuce (Lactuca sativa L.), maize (Zea mays L.), tomato 102
(Lycopersicon esculentum Mill.), and winter wheat (Triticum aestivum L.). The weeds 103
considered as indicator species in this study were common ragweed (A. artemisiifolia L.), 104
large crabgrass (Digitaria sanguinalis (L.) Scop.), barnyardgrass (Echinochloa crus-galli 105
(L.) Beauv.), common purslane (Portulaca oleracea L.) and black nightshade (Solanum 106
nigrum L.). With the exception of A. artemisiifolia, all weed seeds were purchased from
107 Herbiseed1 . 108 109 Laboratory Experiment 110 Experiment 1 111
The potential to inhibit potential of dried common ragweed leaf tissue was studied by 112
assessing its impact on indicator species seed germination and radical and hypocotyl 113
elongation. The experiment was conducted using square plastic “Petri” dishes that 114
contained soil and plant residue mixtures as well as indicator seedlings in a modified 115
Parker bioassay (Weston 2005). Preparation of the experimental sbustrate began with 116
collection of sandy-loam, textured alluvium soil (Typic Udifluvents) from the Tetto Frati 117
research station located in Carmagnola, Italy. Next, the soil was air dried, sifted, and 118
combined with fine silica sand (1:1 v/v) to allow increased water permeability during 119
bioassay preparation. Initially, 100 g of the soil mixture was placed in 100 x 100 x 15 mm 120
Parker dishes. The soil was then topped with varying amounts of chopped common 121
ragweed plant residues (1, 2, or 3 g), after which an additional 50 g of the soil mixture was 122
layered over the residues. Dishes were moistened with 35 ml of deionized water and a 123
square piece of filter paper3 was placed on the soil surface of each dish. The control 124
6
treatment consisted of an inert, prewashed paper towel (1.0 g) cut into about 1.5 mm2 125
pieces combined to the soil mixture. 126
Common ragweed, large crabgrass, barnyardgrass, common purslane, and black 127
nightshade were all used as weed indicator species while alfalfa (cv. Prosementi), lettuce 128
(cv. Meraviglia d’inverno), tomato (cv. San marzano) and winter wheat (cv. Isengrain) were 129
used as crop indicator species. All weed and crop species were exposed to all treatments. 130
Ten seeds of each indicator species were placed uniformly in two separate but parallel 131
rows on the filter paper surface. The dishes were taped shut to maintain moisture and 132
encourage seed residue contact and were stacked and stored at an ambient temperature 133
of 26°C for 6 days in a germination box to promote downward root growth. 134
Total germination was assessed in two ways: a daily count of germinated seeds 135
throughout the experiment and hypocotyl and radical length meaurement after six days. 136
The experiment was arranged as a completely randomized design with four replicates, and 137
the study was replicated twice. While experimental results were analyzed separately for 138
each species, results were combined over runs. 139 140 Greenhouse Experiment 141 Experiment 2 142
Greenhouse experiments were conducted from June through September 2008 in the 143
department experimental greenhouse at temperatures varying between 15 and 25°C. Pots 144
(8 x 8 cm, 8 cm height) were filled with the same soil used for Experiment 1 that was 145
collected at experimental research station Tetto Frati located at Carmagnola, Italy. To 146
each pot was added 1.28 g (equivalent to 2 t /ha) of powdered common ragweed dried 147
tissue residue and then it was thoroughly mixed. The amount used is similar to that used in 148
other allelopathic speicies experiments reported in the literature for Asteraceae (DongZhi 149
7
et al. 2004a; DongZhi et al. 2004b; Hong et al. 2004; Khanh et al. 2005; Tesio et al. 2010; 150
Vidotto et al. 2008). The controls were pots filled with soil only. 151
The impact of common ragweed residues was evaluated based on the germination and 152
growth of several crops: alfalfa, lettuce, tomato, and winter wheat. The seeds were planted 153
in pots devoted to a single indicator species at a rate of 9 seeds for alfalfa and winter 154
wheat and 12 seeds for lettuce and tomato. Immediately after seeding, the pots were 155
watered with a soluble fertilizer (NPK 21-10-20) solution. There after, the pots were 156
watered daily with deionized water to maintain field capacity. The pots were supported by 157
individual flower pot saucers beneath them to prevent contamination from the leaching of 158
other treatments, and arranged on greenhouse benches in a completely randomized 159
design, with four replicates. They were rotated weekly to minimize spatial variation. The 160
experimental unit was the pot, and the experiment was repeated twice. Metal halide lamps 161
supplemented natural light to produce a 14 h day length, which delivered about 55 µmol/s 162
m2. Germination percentage, seedling height, and shoot dry weight were determined 20
163
days after seeding. 164
Experiment 3
165
In early April 2009, pre-germinated seedlings of A. artemisiifolia were transplanted in 166
plastic pots (30 cm height and 27 cm diameter) with a total capacity of about 12 L. Before 167
transplanting, pots were filled with the same soil used in the previous greenhouse 168
experiment. When a 3-leaf stge was reached, the common ragweed pots were thinned to 169
obtain three healthy and uniform plants per pot. Pots were left in an open field in the area 170
of the Dipartimento di Agronomia Selvicoltura e Gestione del Territorio – Grugliasco where 171
they were irrigated twice each week by adding water to the post saucer and weeded 172
weekly by hand. After five months, the common ragweed plants had reached an average 173
fresh weight of 80 g plant-1 (aboveground part) and were carefully removed from the pots
174
with close attention paid to maintain intact roots. Pot soil was then mixed and used as 175
8
substrate for a greenhouse assay, prepared as described below. This experiment was 176
conducted during September 2009 in the experimental glasshouse, under temperatures 177
that varied between 18 and 28°C. Containers (8 x 8 cm, 8 cm height) were filled with soil 178
from Experiment 3 pots used to grow A. artemisifolia during the summer. Control pots 179
were filled with soil maintained under identical conditions as those that contained common 180
ragweed, but where no plants were grown. 181
The potential impact of common ragweed root tissue and exudate was investigated by 182
evaluating the germination and growth of several indicator crop and weed species. The 183
indicator crop species were alfalfa, barley, corn, lettuce, tomato, and winter wheat while 184
the weeds included large crabgrass and barnyardgrass. The number of seeds per pot 185
varied between 4 (maize), 6 (alfalfa, barley, and winter wheat), 9 (barnyardgarss), and 12 186
(large crabgrass, lettuce, and tomato). Only a single indicator species was seeded per pot. 187
Seeded pots were maintained as indicated in Experiment 2. Pots were arranged on 188
greenhouse benches in a completely randomized design with seven replicates, and 189
rotated weekly to reduce spatial variation. The experimental unit was the pot and the 190
experiment was repeated twice. Germination percentage and shoot dry weight were 191
determined 30 days after seeding. 192
Statistical Analysis 193
Both laboratory and greenhouse experiments were analyzed. In the case of the laboratory 194
experiement data, an analysis of variance (ANOVA) was performed separately by species 195
using statistical software SPSS (version 16). In the few cases lacking variance 196
homogenity, a paired samples t-test was conducted to detect differences from the control 197
treatment. After an ANOVA analysis, means were separated using the post-hoc Tukey – b 198
test (p>= 0.05). In the greenhouse studies (Experiments 2 and 3), differences to the 199
control were identified with the independent sample t-test (SPSS software, version 16). 200
9 Results 201 Laboratory Experiment. 202 Experiment 1 203
The effect of common ragweed residues on indicator species seed germination and first 204
seedling growth varied according to indicator species. Results ranged from stimulation to 205
inhibition, especially when the highest rates of dried residue were utilized. 206
Among the weed species, D. sanguinalis seed germination was reduced when common 207
ragweed residues were added at the highest rate (3 g/plate); a 90% reduction in total 208
germination (Table 1) resulted compared to the control. By contrast, the germination 209
percentage of S. nigrum was more than 3 times that of the control when high levels (3 210
g/plate) of residue was used. The root growth of S. nigrum germinated seedlings was 211
reduced by 75% with even the lower amount of residues. Residue addition had neither a 212
simulation nor inhibition effect on weed species shoot growth. The root growth of both A. 213
artemisiifolia and P. oleracea were inhibited about 50% at the highest residue quantities.
214
Among the crops, tomato behaved similar to S. nigrum and showed a germination 215
percentage increase compared to the control (Table 2), even when both shoot and root 216
elongations were depressed by more than 70%. Lettuce suffered growth depression of 217
more than 50% when the highest residue quantities were added to the plate. 218
10 220
Table 1. Effect of different dried leaf tissue concentrations of A. artemisiifolia on total 221
germination (GT), shoot and root length of the weeds: A. artemisiifolia (AMBAR), D. 222
sanguinalis (DIGSA), E. crus-galli (ECHCG), P. oleracea (POROL) and S. nigrum
223
(SOLNI). Mean ± standard error. Values sharing the same letter are not significantly different 224 (Tukey – b test, P<= 0.05). 225 226 Indicator species Control
Dried leaf tissue amounts
(g Parker dish –1)
1.0 2.0 3.0
GT AMBAR 50.0± 7.07a 40.0± 6.41a 42.5± 6.29a 32.5± 2.50a (%) DIGSA 48.8± 0.00a 42.5± 0.15a 30.0± 0.37ab 5.0± 1.34b ECHCG 42.5± 0.00a 62.5± 0.13a 45.0± 0.30a 50.0± 0.42a POROL 55.0± 0.00a 60.0± 0.13a 77.5± 0.23a 60.0± 0.39a SOLNI 12.5± 0.00a 10.0± 0.32a 30.0± 0.37b 45.0± 0.45b
Shoot AMBAR 17.7± 0.26a 16.9± 0.67a 17.9± 1.19a 12.5± 1.04a
(mm) DIGSA 16.8± 0.42a 15.8± 0.75a 17.1± 0.35a 19.5± 0.02a ECHCG 26.5± 1.14a 16.7± 0.59a 27.2± 1.13a 19.9± 0.54a POROL 6.7± 0.47a 9.9± 0.25a 8.45± 0.39a 6.8± 0.26a SOLNI 10.0± 0.83a 6.5± 1.99a 5.9± 0.53a 10.8± 0.63a
Root AMBAR 40.3± 1.10a 37.6± 2.77a 32.6± 5.02ab 21.9± 3.67b
(mm) DIGSA 16.4± 1.73a 17.2± 2.97a 16.3± 1.44a 20.5± 0.02a ECHCG 21.3± 5.85a 13.7± 2.43a 21.1± 5.88a 13.8± 2.39a POROL 9.2± 1.23a 8.1± 0.78ab 7.3± 1.14ab 4.34± 0.68b SOLNI 35.4± 2.62a 8.2± 5.08b 5.5± 1.29b 13.0± 2.08b 227
11 229
Table 2. Effect of different dried leaf tissue concentrations of A. artemisifolia on total 230
germination (GT), shoot and root length of the crops: T. estivum (WHEAT), L. sativa 231
(LETTUCE), M. sativa (ALFALFA), L. esculentum (TOMATO). Mean ± standard error. 232
Values sharing the same letter are not significantly different (Tukey – b test, P<= 0.05). Values 233
marked with * or **are statistically different from the control with p<=0.05 or 0.01. 234
Indicator
species Control
Dried leaf tissues amount
(g Parker dish –1)
1.0 2.0 3.0
GT WHEAT 87.5± 0.00a 97.5± 0.10a 87.5± 0.21a 95.0± 0.00a (%) LETTUCE 97.5± 0.00a 93.7± 0.10a 90.0± 0.21a 85.0± 0.33a ALFALFA 84.4± 0.00a 92.5± 0.10a 92.5± 0.21a 87.5± 0.32a TOMATO 93.7± 0.00 97.5± 0.10 100.0± 0.20* 85.0± 0.33
Shoot WHEAT 41.6± 0.33a 35.2± 0.55a 34.7± 0.23a 35.2± 0.55a
(mm) LETTUCE 24.5± 1.12 10.0± 0.30** 12.0± 0.25* 10.2± 0.35** ALFALFA 20.2± 0.73a 20.4± 0.51a 17.5± 0.44a 20.2± 0.36a TOMATO 22.4± 1.90 15.9± 1.60 13.9± 1.59 6.2± 0.51*
Root WHEAT 48.1± 2.12a 38.2± 3.28a 40.4± 1.37a 38.2± 3.27a
(mm) LETTUCE 26.2± 5.55 13.5± 0.95 12.4± 0.87* 9.8± 1.12* ALFALFA 28.6± 3.29a 27.1± 2.31a 23.9± 1.82a 24.6± 1.61a TOMATO 29.6± 9.00 26.4± 6.40 24.2± 5.92 8.06± 1.27* 235 236 Greenhouse Experiment. 237 Experiment 2 238
Across the seeded crops, total germination (GT) differences relative to the control were 239
observed only for winter wheat (Figure 1), with a germination inhibition of about 30%. The 240
presence of common ragweed residues did not, however, result in a winter wheat plant 241
height reduction (Figure 2) as was true for alfalfa, tomato, and lettuce which showed height 242
reductions of 43%, 41%, and 26%, respectively. All other species showed sensitivity as 243
well. Among the three factors evaluated, total germination, plant height, and plant weight, 244
plant weight displayed the least effect from incorporating A. artemisiifolia residues into the 245
12
substrate (Figure 3). In fact, all crops showed plant weight values of roughly half compared 246
to the control treatment. As in Experiment 1, tomato was reduced the most (58%). 247 248 0 10 20 30 40 50 60 70 80 90
Alfalfa Winter wheat Tomato Lettuce
Tot al ge rm ina ti on (% ) Control AMBAR * 249
Figure 1. Greenhouse experiment: germination response of crop species to the presence 250
of 2 t/ha of common ragweed (AMBAR) dried residues. Bars indicate standard error. 251
* refers to significant differences from the control with p <= 0.05. 252
13 0 5 10 15 20 25
Alfalfa Winter wheat Tomato Lettuce
Pl an t h ei g h t (c m ) Control AMBAR ** ** ** 253
Figure 2. Greenhouse experiment: plant height response of crop species to the presence 254
of 2 t/ha of common ragweed (AMBAR) dried residues. Bars indicate standard error. * 255
refers to significant differences from the control with p <= 0.05 or ** with p <= 0.01. 256 257 0 50 100 150 200 250 300
Alfalfa Winter wheat Tomato Lettuce
Pl an t w ei g h t (m g ) Control AMBAR ** ** * * 258
Figure 3. Greenhouse experiment: plant weight response of crop species to the presence 259
of 2 t/ha of common ragweed (AMBAR) dried residues. Bars indicate standard error. 260
* refers to significant differences from the control with p <= 0.05 or ** with p <= 0.01. 261
14
Experiment 3
263
Germination was not significantly affected if seeds were placed in pots in which A. 264
artemisiifolia was grown (Figure 4). No weed species plant weight was depressed
265
compared to the control treatment, nor were crops lettuce and alfalfa (Figure 5). 266
Conversely, the growth of winter wheat and barley was inhibited by about 60% and 45%, 267
respectively. The high sensitivity of tomato to the allelopathic potential of common 268
ragweed was also confirmed in this experiment; a growth reduction of about 50% was 269
assessed on tomato seedling weight versus the control. 270 271 0 10 20 30 40 50 60 70 80 90 100
DIGSA ECHCG Wheat Lettuce Corn Alfalfa Barley Tomato
Tot al ge rm ina tion (% ) Control AMBAR 272
Figure 4. Greenhouse experiment: germination response to root exudates of common 273
ragweed (AMBAR). Bars indicate standard error. 274
15 0 50 100 150 200 250 300 350 400 450
DIGSA ECHCG Wheat Lettuce Corn Alfalfa Barley Tomato
Pl an t w ei g h t (m g ) Control AMBAR * * * 275
Figure 5. Greenhouse experiment: plant weight response to root exudates of common 276
ragweed (AMBAR). Bars indicate standard error. * refers to significant differences from the 277
control with p <= 0.05. 278
Discussion and conclusion 279
Our findings showed that varying the amount of A. artemisiifolia dried residue in soil 280
affected the seed germination and seedling growth variation of differing indicator crop and 281
weed species. Even though some stimulation effects were observed on germination, 282
growth inhibition was observed more frequently, especially in root growth. 283
In allelopathic experiments, many factors play a role in the bioavailability of the toxic 284
compounds in the rhizosphere, including organic matter adsorption, chemical inactivation 285
and microbial degradation. The Parker dish bioassay used in the laboratory experiment 286
was designed to simulate the allelochemical release into soil by tissue degradation over 287
time after incorporation. This experiment allowed simulation of more realistic conditions 288
than in the natural environment because toxic compound release occured gradually. Our 289
experimental results can, therefore, be compared to those obtained in the greenhouse. 290
16
On average, among the crops, tomato was the most sensitive indicator species to common 291
ragweed allelopathic residues; both in the laboratory and greenhouse experiments, more 292
than 50% growth reduction occurred. The other crops tested showed a different sensitivity 293
to common ragweed. Root exudates showed an inhibitory effect on winter wheat growth 294
that was not observed in the residue degradation experiments. This species showed 295
sensitivity only in the greenhouse experiment. Lettuce dispayed root and shoot growth 296
inhibitory effects only if common ragweed residues were added to the substrate; there 297
were no depressive effects associated with root exudates. 298
Weed species E. crus-galli was never affected by common ragweed, neither in terms of 299
germination nor first seedling growth. Conversely, D. sanguinalis suffered an important 300
germination reduction. From an overall review of the weed species, P. oleracea and S. 301
nigrum showed themselves to be the most sensitive species, with more than a 50% root
302
growth reduction. One explanation of sensitivity to common ragweed of the tested weed 303
species might be related to water imbibition. Indeed, several studies have pointed out that 304
the allelopathic activity of plant residue degradation upon a species that follows is strongly 305
related to seed dimension of the crop (Tesio et al. 2011). The insensitivity observed by E. 306
crus-galli might be explained by its very hard seed coat and associated reduced
307
permeability (Tesio et al. 2008). Even in the case of crops, a similar seed dimension effect 308
could be ascribed as the inhibition effect roughly followed the same rank order as that of 309
seed dimension (tomato > lettuce > winter wheat). 310
No important autotoxic effects were observed as only the root growth of A. artemisiifolia 311
was depressed by residue degradation. 312
The costs to limit the rapid expansion of A. artemisiifolia to Europe’s crops and urban 313
landscape, combined with the economics of pollinosis relief, demand a management 314
response. Presently, eradication is impractical as whether in natural or disturbed settings, 315
17
common ragweed has made clear that it is a widespread and noxious weed of several 316
crops. It has also made evident, in some cases, its ability to colonize the soil rapidly after 317
crop harvest (DiTommaso 2004) Several reports have related the consequence of its 318
presence in the summer after winter wheat or barley cultivations when biomass and root 319
exudates accumulate in the soil and potentially inhibit growth of the succeeding crop 320
(DiTommaso 2004). Awareness of the relationships between this species and other crops 321
and weeds, and knowing that rapid diffusion and expanded presence of the species across 322
southern Europe might be linked to its allelopathic potential, its rapid growth rate, and its 323
remarkable ability to recover from mulching interventions to produce, even after cutting, 324
large amounts of seeds (Patracchini et al. 2011) is useful knowledge for both farmers and 325
the scientific community. 326
Given the potential of common ragweed to reduce crop growth and yields, as well as its 327
costly impact on human health, made a strong case for investigatting the role allelopathy 328
might play in the invasive process. In our studies, germination and initial growth of lettuce 329
and alfalfa were not affected, but winter wheat and tomato were quite sensitive to residue 330
presence. Furthermore, our studies highlighted the cricial concern associated with the 331
sensitivity of the succeeding crop, especially when establishment of a less competitive 332
crop such as tomato is involved. Given these considerations, this work suggests that the 333
management of common ragweed infestation, especially its impact on rotational crops, 334
requires particular attention. In the greenhouse and laboratory experiments we attempted 335
to recreate a possible field situation in which weed biomass is incorporated into the soil 336
and a successive crop is planted. 337
Finally, it should be noted that the greenhouse and laboratory studies conducted here, the 338
effects of residue incorporation and root exudates were evaluated separately. Further 339
studies to consider the synergistic effects of residues and root exudates in greenhouse 340
18
and field settings are necessary to fully determine the impact of this invasive species upon 341
weed and crop establishment in successive cropping systems, as well as the invasion 342
process in natural plant communities. 343
344
Source of material: 345
1 Herbiseed, Twyford, UK.
346
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